Wordpress themes includes database, social, blog and paypal plugins. Whether we need to add them after purchasing
when you want to install theme then at first you have to install wordpress in which a database will create/setup. then you have to install theme and theme bundle plugins and then you have to import demo. in this way in the wordpress database will update with demo data.
Now about social, blog and payment for those you have to choose a theme which will meet your requirements. But blog you can get almost in all themes, payment you can get if you choose a ecomerce/woocommerce compatible theme. and social it can exist in many themes otherwise you can install a social plugin.
Also You can ask your pre-purchase query directly to the theme author by posting comments going through the theme Comments page. theme author will answer your query.